3. The attempt at a solution
For any element [itex]x \in X[/itex], there exists a unique [itex]y \in Y[/itex] for which [itex]F(x) = y[/itex].

Every [itex]n[/itex] element in [itex[X[/itex] will be paired with any one of the [itex]m[/itex] elements in [itex]Y[/itex].
i.e. there exist [itex]m[/itex] possible [itex]F(x_1)[/itex] in [itex]Y[/itex] that can be paired with [itex]x_1[/itex].
[itex]x_2[/itex] can be paired with [itex]m[/itex] possible [itex]F(x_2)[/itex]
[itex]\vdots[/itex]
[itex]x_n[/itex] can be paired with [itex]m[/itex] possible [itex]F(x_n)[/itex].

Because the domain [itex]D_F = X[/itex], every function generated through F will contain [itex]n[/itex] coordinate pairs. Furthermore, since there are [itex]m[/itex] possible values [itex]F(x) = y[/itex] for each element [itex]x[/itex], there are [itex]n[/itex] factors of [itex]m[/itex], or [itex]m^n[/itex], possible functions.